1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory device, and more particularly to a non-volatile semiconductor memory device of the type that reads initialization data stored in a specific region in a memory cell array for initializing (ROM reading) prior to a normal operation (user sequence).
2. Description of the Related Art
A semiconductor memory device such as a NAND-type flash memory requires that voltages in all circuits contained in the semiconductor memory device are retained higher than a minimum operable voltage (hereinafter referred to as a “recovery voltage”), Vccmin, for correct reading, writing (programming) and erasing without errors. For example, a 3V-driven NAND-type flash memory has a recovery voltage Vccmin of about 2.1 V (see FIG. 9A). When a supply voltage lowers below the recovery voltage Vccmin due to an occurrence of peak current, for example, a certain operation (recovery operation) is performed to halt the sequence during execution and initialize the voltages applied to each part for prevention of a malfunction or the like. The sequence once halted restarts when the supply voltage recovers above the recovery voltage Vccmin.
The semiconductor memory device reads out initialization data for various initializations after power is turned on, including redundancy data for replacing failed cells with redundancy cells and trimming data for adjusting timers and voltages, from a storage unit such as a fuse circuit (hereinafter this operation is referred to as “ROM reading”). The semiconductor memory device such as the NAND-type flash memory particularly assigns a specific region in a memory cell array to store the initialization data (see JP-A 2003-178589, for example).
ROM reading is required to start and finish after power is turned on and before the supply voltage reaches a spec-defined lower limit thereof, Vspmin, (for example, Vspmin=2.7 V in the case of the 3V-driven AND-type flash memory (see FIG. 9A)). A value of the supply voltage at the time of ROM reading start, Vrmrd, is hereinafter referred to as a “ROM reading start voltage”. If the spec lower limit of the supply voltage, Vspmin, in the 3V-driven memory is equal to 2.7 V, the ROM reading start voltage Vrmrd can be determined between the recovery voltage Vccmin (2.1 V) and the spec lower limit Vspmin (2.7 V): for example, Vrmrd=about 2.3 V.
During execution of ROM reading, it is required to ensure such a recovery voltage Vccmin that makes the whole circuits in the memory operable. Even If ROM reading starts after the supply voltage once reaches the ROM reading start voltage Vrmrd, an occurrence of peak current may lower the supply voltage below the recovery voltage Vccmin. In this case, the recovery operation is required to halt ROM reading and initialize the voltages applied to each part in the memory cell because the reading error in ROM reading causes an ill effect on operation in the following user sequence.
In the case of the 3V-driven memory (FIG. 9A), a difference (margin) of 0.6 V can be provided between the spec lower limit Vspmin and the recovery voltage Vccmin even in the above numerical example. Therefore, there is little possibility that the supply voltage lowers below the recovery voltage in normal operations (such as reading, programming and erasing).
This margin is inevitably reduced, however, in a memory that is driven under a lower voltage. Accordingly, it is difficult to ensure ROM reading while keeping the spec range of the supply voltage. For example, as shown in FIG. 9B, a 1.8V-driven memory has a recovery voltage Vccmin of about 1.45 V, a spec lower limit Vspmin of about 1.65 V, and a ROM reading start voltage Vrmrd of about 1.5 V between both.
In this case, the ROM reading start voltage Vrmrd and the spec lower limit Vspmin just have a margin of 0.15 V therebetween. Therefore, there is a high possibility that the supply voltage reaches the spec lower limit Vspmin before the completion of ROM reading. An operation by the user such as input before the completion of ROM reading may cause an ill effect, for example, a malfunction possibly. In contrast, if the spec lower limit Vspmin is determined higher to prevent the ill effect, the spec range of the supply voltage is narrowed. If the ROM reading start voltage Vrmrd is determined close to the recovery voltage Vccmin, ROM reading cannot be performed with stability.